The NIH Animal Genetic Resource (NIHAGR) maintains many unique rat genotypes used as models of normal and disease processes in man. Effective management of this resource would be facilitated by the development of embryo cryopreservation procedures for long-term banking. In order to develop banks of cryopreserved germ plasm from laboratory rats, protocols for each step of the embryo banking and rederivation process must be established. These would include the preparation and mating of embryo donor females, embryo collection, cryopreservation, thawing, and embryo transfer. Previous studies suggest that superovulation procedures developed for laboratory mice are not as effective when applied to laboratory rats. The principal objective of this project is to evaluate treatment with an analog of gonadotropin releasing hormone (GnRH-a) as an alternative approach to synchronize estrus cycles for embryo collection and cryopreservation. Our efforts to date have yielded the world's largest bank of cryopreserved rat embryos. A total of approximately 58,000 embryos have been cryopreserved from 109 rat strains and stocks. Seventy rat strains and stocks exist solely as banked embryos. The efficiency of embryo collection varies between models in a manner similar to that of the breeding performance of the model. However, progress has been made in developing banks of cryopreserved embryos from all rat models examined to date. Four recent requests by investigators to reestablish rat models that existed only as frozen embryos demonstrate the usefulness of rat embryo banking. For three of the models, LER/N, LEW/N-op (osteopetrosis), and LEW/N-tl (toothless), the foundation colonies were disbanded after, respectively, 75, 128, and 179 embryos were banked. In the fourth case, M520/N-lr (leaner), a "minimal bank" of 367 embryos was established before the foundation colony was disbanded. Nevertheless, sufficient numbers of pups were obtained after thawing and transfer of, respectively, 23, 94, 55, and 76 embryos to recover breeders to reestablish foundation colonies. In each case, three of the criteria for establishing a bank have been met: pups were rederived from banked rat embryos, rederived pups exhibited the desired genotype, and the rederived pups have bred and produced pups. Embryo banking of these four models to reach our final criteria, 500 banked embryos, is nearly complete.